In recent years, many nonaqueous electrolyte secondary batteries typified by a lithium-ion secondary battery are used as a power source for driving portable electronic devices such as mobile phones, portable personal computers, and portable music players. In addition, emission controls regarding carbon dioxide and the like have been intensified against a backdrop of growing awareness of the need to protect the environment. Therefore, the automotive industry is actively developing not only automobiles using fossil fuels such as gasoline, diesel oil, or natural gas, but also electric vehicles (EV) and hybrid electric vehicles (HEV) that use lithium-ion secondary batteries.
Such lithium-ion secondary batteries use, as a negative electrode active material, a carbon-based material or the like capable of absorbing and desorbing lithium ions, and as a positive electrode active material, a lithium-transition metal composite oxide such as LiCoO2, LiNiO2, or LiMn2O4, and also use an electrolyte that is a solution of a lithium salt dissolved into an organic solvent.
If such a lithium-ion secondary battery is overcharged, an excessive amount of lithium is extracted from a positive electrode and inserted into a negative electrode, and thus, both the positive and negative electrodes are thermally destabilized. The thermal destabilization of both the positive and negative electrodes has an effect of decomposing the organic solvent in the electrolyte over time. Thus, there has been a problem that a rapid exothermic reaction occurs to cause the battery to generate an abnormal amount of heat, resulting in compromising the safety of the battery.
In order to resolve such a problem, a lithium-ion secondary battery has been proposed in which, for example, at least one additive, including biphenyl, cyclohexylbenzene, and diphenyl ether, is added to the electrolyte so as to prevent the temperature from rising when the battery is overcharged (refer to JP-A-2004-134261).
In addition, a lithium-ion secondary battery has been proposed in which the organic solvent of the electrolyte contains an alkylbenzene derivative or cycloalkylbenzene derivative having a tertiary carbon adjacent to a phenyl group so as to ensure safety against overcharging without adversely affecting battery characteristics such as low-temperature characteristics and preservation characteristics (refer to JP-A-2001-015155).
In this lithium-ion secondary battery, when the lithium-ion secondary battery is overcharged, additives, such as cumene, 1,3-diisopropylbenzene, 1,4-diisopropylbenzene, 1-methylpropylbenzene, 1,3-bis(1-methylpropyl)benzene, 1,4-bis(1-methylpropyl)benzene, cyclohexylbenzene, and cyclopentylbenzene, start a decomposition reaction to produce gas, and at the same time, start a polymerization reaction to generate polymerization heat. If the battery continues to be overcharged in this state, the produced amount of gas increases, and then, after 15 to 19 minutes has passed from the start of the overcharge, a current cutting-off sealing plate operates to cut off the overcharging current. As a result, the battery temperature gradually drops.
A non-patent document, “K. Shima et al./Journal of Power Sources 161 (2006) P 1264-1274”, discloses that when a lithium-ion secondary battery containing cyclohexylbenzene in the electrolyte thereof has been overcharged, the cyclohexylbenzene contained in the electrolyte changes to biphenyl, an oligomer of cyclohexylbenzene, or a polymer of cyclohexylbenzene. The non-patent document also discloses that further progress of the reaction forms cross-links between them to produce a modified product of the cyclohexylbenzene.
By incorporating into the electrolyte additives such as cyclohexylbenzene disclosed in JP-A-2001-015155 mentioned above, when a battery has been overcharged, a decomposition reaction of the additives can produce gas to operate a pressure-sensitive current cutoff mechanism, and thus, the safety of the battery can be improved. However, a certain period of time is required from when the battery is in the overcharged state until the pressure-sensitive current cutoff mechanism operates. In addition, depending on the status of battery use, the operation of the current cutoff mechanism may be delayed, for example, when the battery is used under a low-temperature condition. For this reason, the battery temperature can rapidly rise during the period before the current cutoff mechanism operates, thereby causing a film of a separator to melt to break. Thus, a positive electrode plate can be shorted with a negative electrode plate, leading to a thermal runaway.